Methods: Linac modeling actually is a multi-parameter optimization process, and especially depends on the composition and structure detail provided by venders. Maybe a real accelerator use could be substituted by an irregular-inhomogeneous photon surface source with a compensatory contaminative electron source. MLC intensity map is regarded as many grid gather. The source photon is randomly sampled until its original grid intensity more than zero. The source particle weight is just equal to the grid intensity. The transmission direction of the source particle is decided by SID and MLC position just like a point source irradiation. The measurement data of a set of regular fields are used to commission a XH600D 6MV Linac. The 10cm*10cm field PDD is used to affirm the photon energy spectrum. Their half OARs at iso-center surface of multiple regular fields are projected to MLC underside and fitted by Boltzmann function. A set of fitting coefficients are deduced. Their OAR differences before 1.5 cm depth are used to deduce the contaminative electron source. The MLC irregular-inhomogeneous field is divided into several semi-regular sub-fields, whose side intensities and sub-field width are confirmed by the corresponding field coefficients. This algorithm is implemented by adapting the open Monte Carlo code DPM.

Results: This algorithm has been benchmarked with experiment data for regular fields. Basically the difference is under 2%/2mm. The rotational irregular-inhomogeneous multi-fields also are modeled.

Conclusion: A grid intensity-based dose algorithm is presented, which need not know the composition and structure inside a real Linac. This algorithm can simulate the irregular-inhomogeneous field formed by MLC, and is promising for Monte Carlo clinical application.